This relates generally to image sensors, and more specifically, to image sensors with dual photodiode pixels.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include an array of image sensor pixels each of which includes a photodiode and other operational circuitry such as transistors formed in the substrate.
In an effort to enable focus detection, image sensor pixels each of which includes a pair of photodiodes have been developed. An array of microlenses is formed over the image sensor pixels, where each microlens in the array covers a respective pair of photodiodes in a corresponding pixel. A camera lens (sometimes referred to as a macrolens) is formed over the array of microlenses to help direct incoming light to the array of microlenses. When the camera lens is in focus, light will be distributed evenly to each of the two photodiodes in a pixel. When the camera lens is out of focus, however, light will be more concentrated in one of the two photodiodes in a pixel. Thus, during focus sensing operations, the amount of charge can be read independently from the two photodiodes and then compared to determine whether the camera lens is in or out of focus.
This arrangement may, however, be problematic during normal operation when the camera lens is not in focus. Consider, for example, a scenario in which incoming light is out of focus. In such scenarios, charge will predominately accumulate in one of the two photodiodes in a given pixel. When this photodiode becomes full, that photodiode will no longer be sensitive to additional light, and the corresponding output signal will no longer be valid (i.e., the pixel is “saturated”). As a result, the effective maximum pixel capacity will be limited by the storage capacity of each individual photodiode, even if the photodiodes are read out simultaneously and combined. This constraint in pixel capacity can severely limit the image quality of the sensor in bright lighting conditions.
It would therefore be desirable to be able to provide image sensors having dual photodiode pixels with improved pixel capacity.